1. Field of the Invention
This invention relates to a waveform transform method and apparatus, that may be used in a system for efficient image transmission or storage. More particularly, it relates to a waveform transform method and apparatus in which decoding with optional rational number tuple resolution conversion is realized for an input bitstream encoded using wavelet transform encoding.
2. Description of the Related Art
In the conventional representative image compression system, there is a JPEG system (Joint Photographic Coding Experts Group) system, standardized by ISO (International Organization for Standardization). This system, which exploits DCT (discrete cosine transform) to compress and encode mainly a still image, is known to give satisfactory encoded and decoded images subject to allocation of a larger number of bits. However, if, with DCT, the number of bits for encoding is decreased beyond a certain value, block distortion proper to DCT becomes outstanding to render subjective deterioration apparent.
On the other band, investigations are proceeding briskly in an encoding system in which image signals are split into plural bands using a combination of high-pass filters and low-pass filters, termed a filter bank, and are encoded on the band basis. In particular, the wavelet encoding is retained to be promising as a new technique which takes the place of DCT because the new technique is free of a defect, proper to DCT, that block distortion becomes outstanding on higher compression.
Current video appliances, such as electronic still cameras or video movies, exploit JPEG or MPEG (Moving Picture Image Coding Experts Group) as the picture or image compression system, and DCT as the transform system. It is, however, premeditated that a product exploiting the transform system, which is based on the wavelet transform, will make its debut on the market.
However, there has not been presented an invention targeted at furnishing a product exploiting the feature of the wavelet transform, even although investigations towards improving the efficiency in the encoding system are going on energetically.
In the conventional wavelet transform and inverse transform, it is retained that, due to the properties of these transforms, resolution can be lowered or raised, by way of scaling, only in terms of powers of 2. It may, however, be premeditated that, if the resolution of an original image becomes higher, an increasing demand may be raised for decoding an image with a resolution other than powers of 2. That is, if decoding is possible with the resolution of optional rational number including powers of 2, limitations imposed by the terminal side become no longer decisive, thus leading to an appreciably enlarged field of application.
It is therefore an object of the present invention to provide a wavelet decoding method and apparatus in which image signals encoded for compression using the wavelet transform as the transform system can be decoded to a resolution of an optional rational number without being influenced by constraints imposed by the terminal side, as a result of which so-called thumb-nail images or images obtained on resolution conversion of an original image (contracted or enlarged image) frequently used in electronic still cameras or printers, can be stored or displayed efficiently to enlarge the field of application to a wide variety of products.
In one aspect, the present invention provides a entropy decoding means for entropy decoding an encoded bitstream to transmit quantized coefficients, dequantizing means for dequantizing the quantized coefficients to transmit transform coefficients, transform coefficient back-scanning means for scanning the transform coefficients in a pre-set fashion to re-array the transform coefficients, and inverse wavelet transform means for inverse transforming the re-arrayed transformation coefficients to furnish a decoded image. The inverse wavelet transform means adaptively constitute an upsampler, a downsampler and a synthesis filter in dependence upon a pre-set resolution conversion factor.
In another aspect, the present invention provides a entropy decoding means for entropy decoding an encoded bitstream to transmit quantized coefficients, dequantizing means for dequantizing the quantized coefficients to transmit transform coefficients, transform coefficient back-scanning means for scanning the transform coefficients in a pre-set fashion to re-array the transform coefficients, and inverse wavelet transform means for inverse transforming the re-arrayed transformation coefficients to furnish a decoded image. The inverse wavelet transform means converts the resolution of an inverse transformed image, higher in resolution than an image obtained with a pre-set resolution conversion factor, by the combination of a digital filter, a downsampler or an upsampler, taken singly or in combination.
In yet another aspect, the present invention provides a entropy decoding an encoded bitstream to transmit quantized coefficients, dequantizing the quantized coefficients to transmit transform coefficients, scanning the transform coefficients in a pre-set fashion to re-array the transform coefficients and inverse transforming the re-arrayed transformation coefficients to furnish a decoded image. In the inverse wavelet transform, the inverse wavelet transform means converts the resolution of an inverse transformed image, higher in resolution than an image obtained with a pre-set resolution conversion factor, by the combination of a digital filter, a downsampler or an upsampler, taken singly or in combination.
The upsampling has the effect of raising the resolution. Specifically, zero-value pixels are interpolated. On the other hand, downsampling has the effect of lowering the resolution. Specifically, sampling is effected every several pixels. The digital filtering, expressed by a suitable transfer function, performs processing every several pixels and multiplexes filter coefficients (impulse response) every pixel and executes convolution.
In the wavelet decoding device and method of the present invention, upsampling, downsampling and synthesis filtering are adaptively performed in inverse wavelet transform in dependence upon the pre-set resolution conversion factor and, on the downstream side of the inverse wavelet transform, upsampling and filtering for synthesis are carried out until a synthesized image of a pre-set enlarging factor is obtained. Also, in inverse wavelet transform, an inverse-transformed image having a resolution higher than that of the image obtained with a pre-set resolution conversion factor is resolution-converted by digital filtering, downsampling and upsampling, taken singly or in combination. In this manner, an image encoded for compression using a wavelet transform as the transform system can be decoded with a resolution a corresponding to an optional rational number, as a result of which a so-called thumbnail image used frequently in e.g., an electronic still camera or a printer, or an image resolution-converted from an original image (contracted or enlarged image) can be stored or displayed efficiently, thus enabling the application to various products to be expanded appreciably.
That is, according to the present invention, in which a band image eventually stored in an image memory can be displayed as a thumbnail image or a contracted image on a screen, if the process of generating a band-split image and the encoding process are designed as a common process, the processing can be improved in efficiency. Therefore, it is unnecessary to provide a particular circuit for generating a thumbnail image, thus reducing the hardware scale. Moreover, if an external storage device, for example, is annexed to the present device for storage and holding of the encoded bitstream, encoded bitstreams of a large number of images can be stored and held in the external storage device. Also, since the thumbnail images or contracted images need not perpetually be stored and held on the image memory, it suffices if the encoded bitstream of the thumbnail image or the contracted image desired to be viewed is incidentally read out, decoded and displayed on the screen, thus improving the utilization efficiency.